To obtain hydrocarbons such as oil and gas, wellbores (also referred to as the boreholes) are drilled by rotating a drill bit attached at the end of a drilling assembly generally referred to as the “bottom hole assembly” (BHA) or the “drilling assembly.” The wellbore path of such wells is carefully planned prior to drilling such wellbores utilizing seismic maps of the earth's subsurface and well data from previously drilled wellbores in the associated oil fields. Due to the very high cost of drilling such wellbores and the need to minimize time actually spent drilling and wireline logging wells, it is essential to gain as much information as possible during drilling of the wellbores. Information about downhole conditions and materials may be acquired with wireline tools or bottom hole assemblies (BHA). Wireline tools are generally used after a wellbore is drilled, bottom hole assemblies may be used while the well is being drilled as part of the drilling string. Downhole wellbore information acquired from BHA components may be utilized, among other things, to monitor and adjust the drilling direction of the wellbores or to detect the presence of geologic formations and hydrocarbons.
In logging while drilling through an earth formation, it is desirable to measure formation shear wave velocity. The shear wave velocity of earth formations provides information important for exploration and production of oil and gas from the formation. The shear wave velocity profile enables the conversion of seismic shear wave time sections to depth sections and is utilized in the interpretation of seismic wave amplitude variation versus detector offset. The ratio between the shear wave velocity and the compressional wave velocity is closely related to the rock lithology and is related to hydrocarbon saturation. Shear wave velocity is also used to evaluate the mechanical properties of the formation in reservoir engineering applications.
Because of the importance of earth formation shear velocity, various methods have been developed to measure it. In conventional wireline logging using a monopole acoustic tool, the shear velocity can be measured from the shear wave refracted along the borehole wall if the formation shear wave velocity is greater than the borehole fluid acoustic velocity. A formation that has a shear wave velocity faster than the borehole fluid is called a ‘fast formation.’ However, in a formation where the shear velocity is slower than borehole fluid velocity, a ‘slow formation,’ the shear wave can no longer refract along the borehole wall, and the shear velocity cannot be directly measured from monopole logging. Because of the need to measure shear velocity in slow formations, especially in the soft sediments of deep-water reservoirs, dipole acoustic logging tools were developed. The dipole tool induces and measures the bending or flexural wave motion in the formation. In a sufficiently low frequency range (1-3 kHz), the flexural wave travels at the shear velocity of the formation, regardless whether the formation is fast or slow. This allows for direct measurement of formation shear velocity using the dipole acoustic tool. Dipole acoustic logging is now a mature technology with worldwide commercial applications.
A viable technique for shear wave velocity measurement is using the quadrupole shear waves. A quadrupole acoustic tool induces and measures the quadrupole shear wave in the formation. The low-frequency portion of the wave travels at the formation shear wave velocity, allowing for direct shear velocity measurement from the quadrupole wave. Although the quadrupole shear wave has been extensively studied theoretically and a wireline quadrupole-logging tool was also proposed (Winbow et al., 1991 in U.S. Pat. No. 5,027,331), this technology has not yet been commercially applied to the oil and gas industry. This is largely because the wide acceptance and success of the dipole shear wave technology have fulfilled the needs for measuring shear velocity in slow formations.
The acoustic Logging-While-Drilling (LWD) technology has been developed in recent years out of the needs for saving rig-time and for real-time applications such as geosteering and pore pressure determination, among others. The LWD acoustic technology is aimed at measuring the compressional- and shear-wave velocities of an earth formation during drilling. This technology has been successful in the measurement of compressional wave velocity of earth formations. The need for determining the shear wave velocity in slow formations calls for further development of the technology for shear wave measurement capability. Because of the popularity and success of the dipole shear wave technology in wireline logging, this technology is naturally extended to the LWD situation and a LWD dipole acoustic tool has been built and offered for commercial applications.
The application of the dipole acoustic technology to LWD has a serious drawback caused by the presence of the drilling collar with BHA that occupies a large part of the borehole. The drawback is that the formation dipole shear wave traveling along the borehole is severely contaminated by the dipole wave traveling in the collar. There is a need for a method of determination of shear wave velocities of earth formations that is relatively robust in the presence of tool mode waves propagating along the drill collar. The need is particularly acute in situations where the formation shear velocity is less than the velocity of propagation of compressional waves in borehole fluids. The parent application presents a system and methods for using higher acoustic modes, such as, for example, the quadrupole mode for determining the formation shear velocity. As discussed in the parent application and later, herein, the quadrupole, and higher, modes exhibit a cut-off frequency in the drill collar below which, these higher modes do not propagate in the collar. Therefore, it is desirable to transmit signals into the formation at frequencies below the collar cut-off frequency. One range of desirable frequencies, for example, is 1-3 kHz.
Common downhole acoustic sources utilize piezoelectric transducers for generating the acoustic signals. While such transducers may exhibit acceptable signal strength at higher frequencies, for example >10 kHz, they are typically less efficient at lower frequencies of interest for the investigations desired here. The low signal strength can be masked by the drilling noise present during drilling. Low signal strength also limits the depth of investigation for such a system.
There is a need for an acoustic signal generator that provides a relatively high signal strength over the entire frequency range of interest for acoustic logging while drilling investigations.